Sapphire or “sapphire glass,” as it is sometimes called, is a ceramic that has many industrial applications, from watch covers to envelopes for use in high temperature lamps. It is also used in military applications. Covers used in many electronic devices today, such as displays, require not only transparency but hardness for anti-scratch capability. Sapphire, one of the hardest materials, is an ideal material to meet this need. Recently, sapphire or single crystalline Al2O3 has been referred to as “sapphire glass” which is a layman's term meant to highlight the fact that crystalline Al2O3 is transparent like glass. As a transparent material with a hardness only second to diamond, it has been claimed recently as an ideal material for display covers. But sapphire glass has in fact long been used in the semiconductor industry for various applications, along with other industries such as the watch industry, for just one example. The one and only drawback of sapphire, at least as far as scratch-resistance goes, has been cost. Recently there have been attempts to reduce the price of sapphire glass for use as display covers by GTAT and Apple Inc., that disclosed methods for making inexpensive sapphire glass. Apple has disclosed additional technology for sapphire glass covers in patent applications, as have other companies, such as Corning Inc. “Sapphire glass” should therefore be considered to include polycrystalline or nanocrystalline Al2O3 (not just single crystalline) given recent patent disclosures by Apple Inc.
Industrial sapphire is created by melting aluminum oxide (Al2O3) at 2040° C. and then encouraging crystal growth with a seed and careful control of the environment. Manufacturers have developed several unique methods for growth, with varying levels of resultant quality, size, and cost. The EFG or Stephanov methods allow the directed growth of shapes like ribbon, or even tubes, however there are many limitations to what can be done. The Czochralski, HEM, or Kiropolous methods allow the highest optical quality sapphire, but the result is a rod-like “blob” of crystal called a boule, that must be entirely machined into useable shapes and sizes. Traditionally sapphire glass has been manufactured by forming boules by either the Verneuil or Czochralski processes and then slicing the sapphire from these boules. However, this method requires very high temperatures and cutting and polishing the sapphire boules requires added time and process challenges. More to the point, when making sapphire glass for devices such as smartphones, or other small devices, sapphire ingot yield rates can be as low as below 50 percent. For these and other reasons, sapphire glass as it is currently produced is expensive and not economical. Alternatively, one can make sapphire glass by sintering Al2O3 powder in order to form small grain Al2O3 material. Crystalline Al2O3 made from small grains is known to be as hard and potentially even harder than single crystal sapphire or sapphire glass. However, this sintering process must also be performed at very high temperature, greater than 1200° C., and the process is also quite involved and so far has not been a commercially viable solution to making inexpensive sapphire glass. Recently, an invention for improving sapphire glass manufacturing was disclosed by Chaudhari et al (see US 2014/0116329) and “Extremely highly textured MgO [111] crystalline films on soda-lime glass by e-beam” (Materials Letters 121 (2014) 47-49). These disclosures fail, however, to provide a method for making an enhanced quality ceramic (e.g. sapphire) layer on the crystalline MgO substrate.
Thus, a new method is disclosed here that will not only provide sapphire glass that is cost effective, simple and can take place at low temperatures (ideally 600 C or below), and also provide small grains for added hard, scratch-free, material, but can provide an enhanced, high quality ceramic layer such as sapphire In accordance with one aspect of the present invention, the foregoing and other objects can be achieved by using the common electron beam (e-beam) evaporation process known in the trade, and depositing Magnesium Oxide (MgO) on a soda-lime glass substrate.
In accordance with another aspect of the present invention the foregoing and other objects can be achieved by using e-beam evaporating Al in an O2 atmosphere to get a crystalline film on the MgO layer previously deposited. Specifically, this is done by evaporating Al and then adding O2 so that Al reacts with O2 on the surface of the MgO to form crystalline oxide.
In accordance with another aspect of the present invention the foregoing and other objects can be achieved by keeping Al on the MgO surface so it can spread on the surface to form a desired crystalline phase.